Pi control of the PHO regulon is a form of transmembrane signal transduction. Pi control requires PhoU, PhoB, PhoR, and the binding protein-dependent PstSCAB, Pi transporter. Both the pstSCAB-phoU and phoBR operons are derepressed 100-fold during Pi limitation. The Pst transporter and PhoU have negative regulatory roles. Apparently, a signal for Pi repression flows from the Pst transporter when all PstSCAB complexes are saturated with Pi, regardless of how few (repressed condition) or many (derepressed condition) complexes are present. This signal is somehow communicated, perhaps via PhoU, to PhoR which in turn acts as the Pi sensor. PhoB and PhoR are members of the large family of two-component regulatory systems in bacteria. PhoB is a DNA-binding protein and transcriptional activator. PhoR is an autophosphorylase, a kinase for PhoB, and (presumably) a phosphatase for PhoB-Pi; it converts PhoB between its transcriptionally active (PhoB-Pi) and inactive (free PhoB) forms. The phoBR operon is unusual because it is subject both to transcriptional and translational controls; the latter appears to involve a form of antisense RNA control that is physiologically regulated. This proposal concerns the mechanism of Pi transmembrane signal transduction involving the PstSCAB transporter, PhoU, and PhoR, and control of the phoBR operon. Molecular genetic studies are designed to define what components are involved in Pi control and determining how they interact. Biochemical studies on wild-type and mutant proteins are designed to detect critical protein-protein interactions that are predicted from an analysis of suppressor mutations. Studies on the expression of the pstSCAB-phoU and phoBR operons are designed to determine how the amounts of key gene products affects Pi control and what significant regulatory signals occur within each operon. These studies on Pi control of the PHO regulon are designed to contribute not only to our understanding of how cells regulate genes for P assimilation but also how cells recognize environmental stimuli in general.